widget attributes

Description

There are at least two attributes (disabled and readonly, anything else?) for misc. form fields that don't have parameter. Because attrs is a dictionary, every key needs a value. When value is set to None, it finally renders to attribute="None":

There should be a way to pass attributes without value (maybe using None as a value?). Current e.g. <input disabled="1" name="fred" value="stone"> works under every browser I was able to test, but it doesn't change the fact it's not valid html.